Patients over age 45 who have had a stroke or transient ischemic attack (TIA) within 1 to 4 months of this study may be eligible to participate. Candidates will be screened with a review of their past medical records and neurologic and medical evaluations that may include magnetic resonance imaging (MRI) of the brain, ultrasound or magnetic resonance angiography (a type of MRI) of the carotid arteries (arteries in the neck that supply blood to the brain), echocardiography (ultrasound test of the heart), electrocardiography (EKG) and blood tests.